Caster wheels are commonly used on mobile equipment carriers to facilitate movement of the equipment from place to place. The wheels on such caster wheel suspensions are subjected to relatively high impact forces when they engage an obstruction, which impact forces increase as a function of increasing speed and weight or load, and decreasing wheel radius. Shock absorbing caster wheel suspensions such as disclosed in U.S. Pat. Nos. 2,738,542 and 3,518,714 have heretofore been provided to reduce transmission of impact forces from the ground engaging wheels to the mobile equipment supported on the equipment carrier.
Mobile equipment carriers are sometimes transported on freight carriers such as trucks, airplanes and the like with the carriers supported by the shock absorbing wheel suspensions. The shock absorbing wheel suspensions function to absorb shock loads transmitted by the freight carrier to the wheel of a shock absorbing wheel suspensions and dynamic or "G" loads transmitted by the load or equipment to the shock absorbing wheel suspensions. However, when shock absorbing wheel suspensions are used to support the load on a freight carrier, it has been found that the equipment carrier sometimes goes into heavy vibration which can be damaging to some types of goods and equipment. Thus, while shock absorbing wheel suspensions are adapted to absorb and reduce transmission to the equipment of relatively high impact shocks of a random nature, a periodic disturbing force from the freight carrier, even of low amplitude, can produce very large forced vibration in the equipment supported on the shock absorbing wheel suspension when the frequency of the disturbing force approaches the frequency of natural vibration of the shock absorbing wheel suspension and the goods or equipment supported thereby.
The applicant's prior U.S. Pat. No. 4,685,174 discloses a shock absorbing caster suspension for a mobile equipment carrier wherein each shock absorbing caster includes a caster bracket mounted on the equipment carrier for swiveling movement about an upright caster axis, a wheel support bracket mounted on the caster bracket for swinging movement about a horizontal pivot axis, a caster wheel mounted on the wheel support bracket for rotation about a horizontal wheel axis parallel to the pivot axis, and a vibration dampening means acting between the wheel support bracket and the caster bracket for dampening brake oscillating of the wheel support bracket relative to the caster bracket. When transporting mobile equipment on freight carriers, it is of course necessary to anchor the mobile equipment against horizontal movement relative to the freight carrier. However, in order to achieve the benefits of shock absorption and vibration dampening provided by the above-described shock absorbing wheel suspension, the anchor must accommodate vertical movement of the mobile equipment carrier while inhibiting horizontal movement. One anchor system utilizes flexible straps attached to the mobile equipment and anchored to bulkheads on the freight carrier in a manner to resist horizontal movement of the mobile carrier while accommodating limited vertical movement. Such an anchoring system is quite cumbersome since it requires a plurality of straps to effectively anchor the mobile equipment against horizontal movement in all directions. It has also been proposed for example as disclosed in U.S. Pat. No. 3,066,950 to extend the caster wheel axles and anchor the axles to a platform such as the bed of a truck. Extension of the wheel axles not only requires modification of the construction of the caster wheel suspensions, but can also limit swinging movement of the wheel support bracket relative to the caster bracket and thus adversely affect the shock absorbing characteristics of the caster wheel suspension.